The present invention relates to a novel process for the preparation of (1R,4S)xe2x80x94 or (1S,4R)-1-amino-4-(hydroxymethyl)-2-cyclopentene of the formulae 
or salts thereof, or the D- or L-hydrogentartrates thereof and also their further conversion to give (1S,4R)xe2x80x94 or (1R,4S)-4-(2-amino-6-chloro-9-H-purine-9-yl)-2-cyclopentene. (1R,4S)-1-Amino-4-(hydroxymethyl)-2-cyclopentene of the formula IV is an important intermediate for the preparation of carbocyclic nucleosides such as, for example, Carbovir(copyright) (Campbell et al., J. Org. Chem. 1995, 60, 4602-4616).
A process for the preparation of (1R,4S)-1-amino-4-(hydroxymethyl)-2-cyclopentene is described, for example, by Campbell et al. (ibid) and by Park K. H. and Rapoport H. (J. Org. Chem. 1994, 59, 394-399). In this process, the starting material is either D-glucono-xcex4-lactone or D-serine, approximately 15 synthesis stages being required to form (1R,4S)-N-tert-butoxycarbonyl-4-hydroxymethyl-2-cyclopentene, and the protecting group is removed to give (1R,4S)-1-amino-4-(hydroxymethyl)-2-cyclopentene.
Both these processes are costly, complex and not practicable industrially. WO 93/17020 describes a process for the preparation of (1R,4S)-1-amino-4-(hydroxymethyl)-2-cyclopentene, in which (1R,4S)-4-amino-2-cyclopentene-1-carboxylic acid is reduced to the desired product using lithium aluminium hydride. Disadvantages of this process are firstly that the double bond of the cyclopentene ring is also reduced, the poor handling properties of lithium aluminium hydride and secondly that it is too costly.
Taylor S. J. et al. (Tetrahetron: Asymmetry Vol. 4, No. 6, 1993, 1117-1128) describe a process for the preparation of (1R,4S)-1-amino-4-(hydroxymethyl)-2-cyclopentene starting from (xc2x1)-2-azabicyclo[2.2.1]hept-5-en-3-one as starting material. In this process, the starting material is converted, using microorganisms of the species Pseudomonas solanacearum or Pseudomonas fluorescens, into (1R,4S)-2-azabicyclo[2.2.1]hept-5-en-3-one, which is then reacted with di-tert-butyl dicarbonate to give (1R,4S)-N-tert-butoxycarbonyl-2-azabicyclo[2.2.1]hept-5-en-3-one, and the latter is reduced using sodium borohydride and trifluoroacetic acid to give the desired product. This process is far too costly.
In addition, Martinez et al. (J. Org. Chem. 1996, 61, 7963-7966) describe a 10-stage synthesis of (1R,4S)-1-amino-4-(hydroxymethyl)-2-cyclopentene starting from diethyl dialkylmalonate. This process too has the disadvantage that it is complex and not practicable industrially.
It is also known that N-substituted (xc2x1)-2-azabicyclo-[2.2.l]hept-5-en-3-ones, which carry an electron-with-drawing substituent, can be reduced to the corresponding N-substituted aminoalcohols using a metal hydride (Katagiri et al., Tetrahedron Letters, 1989, 30, 1645-1648; Taylor et al., ibid).
In contrast to this, it is known that unsubstituted (xc2x1)-2-azabicyclo[2.2.1]hept-5-en-3-one of the formula 
is reduced with lithium aluminium hydride to give (xc2x1)-2-azabicyclo[2.2.2]octene (Malpass and Tweedle, J. Chem. Soc., Perkin Trans 1, 1977, 874-884), and that the direct reduction of (xc2x1)-2-azabicyclo[2.2.2]hept-5-en-3-one to give the corresponding aminoalcohol has to date been impossible (Katagiri et al., ibid; Taylor et al., ibid).
It is also known to resolve racemic 1-amino-4-(hydroxymethyl)-2-cyclopentene using (xe2x88x92)-dibenzoyltartaric acid (U.S. Pat. No. 5,034,394). On the one hand, this reaction has the disadvantage that (xe2x88x92)-dibenzoyltartaric acid is expensive, and, on the other hand, that the separation must take place in the presence of an exactly defined mixture of acetonitrile and ethanol. This solvent mixture cannot be removed and must be fed to the combustion.
The object of the present invention was to provide a simple, economical and cost-effective process for the preparation of a (1R,4S)-1-amino-4-(hydroxymethyl)-2-cyclopentene.
Surprisingly, it has now been found that when (xc2x1)-2-azabicyclo[2.2.1]hept-5-en-3-one of the formula 
in the form of the racemate or one of its optically active isomers, is reduced with a metal hydride, the aminoalcohol of the formula 
in the form of the racemate or one of its optically active isomers is obtained in a simple manner.
As the person skilled in the art is aware, the aminoalcohol of the formula I can be converted using an acid into the corresponding salts, such as, for example, into hydrohalide salts. Suitable hydrohalide salts are hydrobromides and hydrochlorides.
The starting material, the (xc2x1)-2-azabicyclo-[2.2.1]hept-5-en-3-one can be prepared according to EP-A 0 508 352.
Metal hydrides which may be used are alkali metal or alkaline earth metal hydrides and also binary or complex metal hydrides of the boron or aluminium group, such as alkali metal and alkaline earth metal borohydrides, alkali metal and alkaline earth metal aluminium hydrides. Suitable alkali metal or alkaline earth metal hydrides are LiH, NaH, KH, BeH2, MgH2 or CaH2.
Binary alkali metal or alkaline earth metal borohydrides which may be used are NaBH4, LiBH4, KBH4, NaAlH4, LiAlH4, KAlH4, Mg(BH4)2, Ca(BH4)2, Mg(AlH4)2 and Ca(AlH4)2. Complex metal hydrides of the boron or aluminium group may have the general formula M1M2HnLm, in which n is an integer from 1 to 4, and m is an integer from 4 to 4 minus the corresponding number n, M1 is an alkali metal atom, M2 is boron or aluminium, and L is C1-4-alkyl, C1-4-alkenyl, C1-4-alkoxy, CN or an amine, or the complex metal hydrides may have the general formula M2HOLp, in which M2 is as defined above and O is an integer from 0 to 3, and p is an integer from 3 to 3 minus the corresponding number p. Possible M1M2HnLm compounds are LiBH(C2H5)3, LiBHx(OCH3)4xe2x88x92x, LiAlH(OC(CH3)3)3, NaAlH2 (C2H4OCH3)2, NaAlH2(C2H5)2 or NaBH3CN. Preferably, the reduction is carried out using a metal borohydride. As an expert in the art is aware, the metal hydrides mentioned such as, for example, LiBH4, can also be produced xe2x80x9cin situxe2x80x9d. Common preparation methods for LiBH4 are, for example, the reaction of an alkali metal borohydride with a lithium halide (H. C. Brown et al., Inorg. Chem. 20, 1981, 4456-4457), the reaction of LiH with B2O3 in the presence of hydrogen and a hydrogenation catalyst (EP-A 0 512 895), the reaction of LiH with (H5C2)OBF3 (DE-A 94 77 02) and that of LiH with B(OCH3)3 (U.S. Pat. No. 2,534,533).
The metal hydrides are expediently used in a molar ratio of from 1 to 5 per mole of (xc2x1)-2-azabicyclo-[2.2.1]hept-5-en-3-one.
The metal hydrides, in particular NaBH4, are preferably used with lithium salt additives. Lithium salts which may be used are LiCl, LiF, LiBr, LiI, Li2SO4, LiHSO4, Li2CO3, Li(OCH3) and LiCO3.
The reduction is expediently carried out in an inert-gas atmosphere, such as, for example, in an argon or nitrogen atmosphere.
The reduction can be carried out at a temperature of from xe2x88x9220 to 200xc2x0 C., preferably at a temperature of from 60 to 150xc2x0 C.
Suitable solvents are aprotic or protic organic solvents. Suitable aprotic organic solvents may be ethers or glycol ethers, such as, for example, diethyl ether, dibutyl ether, ethyl methyl ether, diisopropyl ether, tert-butyl methyl ether, anisole, dioxane, tetrahydrofuran, monoglyme, diglyme and formaldehyde dimethylacetal. Suitable protic organic solvents are C1-6-alcohols, such as methanol, ethanol, propanol, isopropanol, butanol, tert-butanol, pentanol, tert-amyl alcohol or hexanol and also mixtures of these with water. Suitable protic organic solvents are also mixtures of one of said ethers, glycol ether with water or with one of said alcohols, such as a mixture of a C1-6-alcohol with an ether or glycol ether, in particular a mixture of methanol, ethanol or water with diethyl ether, tetrahydrofuran, dioxane, glyme or diglyme. The solvent used is preferably a protic organic one, such as a mixture of a C1-6-alcohol or water with an ether or glycol ether.
In a preferred embodiment, the reduction is carried out in the presence of an additive, such as in the presence of water or of a lower aliphatic alcohol. The lower aliphatic alcohol may be methanol, ethanol, methoxyethanol, n-propanol, isopropanol, isobutanol, tert-butanol, n-butanol, diols such as butanediol, and triols such as glycerol. In particular, the lower aliphatic alcohol is methanol or ethanol. Here, the lower aliphatic alcohol is expediently used in a molar ratio of from 2 to 15 per mol of (xc2x1)-2-azabicyclo[2.2.1]hept-5-en-3-one.
If the reaction is carried out in the presence of said alcohol, the corresponding amino acid ester can be formed in situ (intermediate). I.e. if the starting material used is (xc2x1)-2-azabicyclo[2.2.1]hept-5-en-3-one, according to the invention the corresponding (xc2x1)-amino acid ester can be formed. If the starting material used is (xe2x88x92)-2-azabicyclo[2.2.1]hept-5-en-3-one, according to the invention the (xe2x88x92)-amino acid ester can correspondingly be formed as intermediate.
Surprisingly, it has also been found that when a cyclopentene derivative of the general formula 
in the form of the racemate or one of its optically active isomers, in which R is C1-4-alkyl, C1-4-alkoxy, aryl or aryloxy, is hydrolyzed with an alkali metal hydroxide, the aminoalcohol of the formula 
in the form of the racemate or one of its optically active isomers is obtained in a simple manner.
C1-4-Alkyl can be substituted or unsubstituted. In the text below substituted C1-4-alkyl is taken to mean C1-4-alkyl substituted by a halogen atom. The halogen atom may be F, Cl, Br or I. Examples of C1-4-alkyl are methyl, ethyl, propyl, butyl, isobutyl, tert-butyl, isopropyl, chloromethyl, bromomethyl, dichloromethyl and dibromomethyl. The C1-4-alkyl is preferably methyl, ethyl, propyl, butyl, isobutyl or chloromethyl.
The C1-4-alkoxy used may be, for example, methoxy, ethoxy, propoxy or butoxy. The aryl used can be, for example, phenyl or benzyl, substituted or unsubstituted. The aryloxy used can be, for example, benzyloxy or phenoxy, substituted or unsubstituted.
The alkali metal hydroxide used may be sodium or potassium hydroxide.
For this process variant, the cyclopentene derivative of the general formula III is preferably prepared by reduction of the corresponding acyl-2-aza-bicyclo[2.2.1]hept-5-en-3-one of the general formula 
in the form of the racemate or one of its optically active isomers, in which R is as defined above, using one of the metal hydrides already mentioned in an anhydrous solvent.
The anhydrous solvent may be protic or aprotic organic solvents, in particular an anhydrous protic organic solvent such as a tertiary alcohol. The tertiary alcohol may be tert-butyl alcohol or tert-amyl alcohol.
As already mentioned above, this reduction is also preferably carried out in the presence of an addition, such as in the presence of a lower aliphatic alcohol such as methanol, in particular in the presence of 2 mol of methanol per mole of acyl-2-azabicyclo-[2.2.1]hept-5-en-3-one (formula IV).
The reaction is expediently carried out at a temperature of from 0 to 50xc2x0 C., preferably from 15 to 30xc2x0 C.
The racemic aminoalcohol of the formula I is then converted according to the invention either by chemical means using an optically active tartaric acid or by biotechnological means using a hydrolase in the presence of an acylating agent to give (1R,4S)xe2x80x94 or (1S,4R)-1-amino-4-(hydroxymethyl)-2-cyclopentene of the formula 
or salts thereof and/or to give (1S,4R)xe2x80x94 or (1R,4S)-1-amino-4-(hydroxymethyl)-2-cyclopentene derivative of the general formulae 
or salts thereof, in which X and Y are identical or different and are an acyl group or H, with the exception of Xxe2x95x90Yxe2x95x90H.
The hydrolases used may be lipases, proteases, amidases or esterases, lipases being expediently used.
In the text below, salts are taken to mean hydrohalide salts such as hydrochlorides, hydrobromides or tartrates.
As the person skilled in the art is aware, hydrolase-catalyzed acylations in which optically active compounds are formed are carried out in the presence of a suitable acylating agent (Balkenhohl et al., 1997, J. Prakt. Chem. 339, 381-384; K. Faber, xe2x80x9cBiotransformation in Organic Chemistryxe2x80x9d, 2nd ed., Berlin 1995, 270-305). Suitable acylating agents are generally carboxylic acid derivatives such as carboxamides, carboxylic anhydrides or carboxylic esters. The carboxylic esters may, for example, be alkoxycarboxylic esters, such as ethyl methoxyacetate and isopropyl methoxyacetate, C1-6-carboxylic esters, such as butyl acetate, ethyl butyrate and ethyl hexanoate, glyceryl esters, such as tributyrin (glyceryl tributyrate), glycol esters, such as glycol dibutyrate and diethyl diglycolate, dicarboxylic esters, such as diethyl fumarate and malonate, cyanocarboxylic esters, such as ethyl cyanoacetate, or cyclic esters, such as, for example, 6-caprolactone.
Accordingly, the acyl group in the formulae VII and VIII corresponds to the acid component of the carboxylic acid derivative used.
The lipases used may be standard commercial lipases, such as, for example: Novo lipase SP523 from Aspergillus oryzae (Novozym 398), Novo lipase SP524 from Aspergillus oryzae (lipase=Palatase 20000 L from Novo), Novo lipase SP525 from Candida antarctica (lipase B Novozym 435, immobilized), Novo lipase SP526 from Candida antarctica (lipase A=Novozym 735, immobilized), lipase kits from Fluka (1 and 2), Amano P lipase, lipase from Pseudomonas sp., lipase from Candida cylindracea, lipase from Candida lypolytica, lipase from Mucor miehei, lipase from Aspergillus niger, lipase from Bacillus thermocatenulatus, lipase from Candida antarctica, lipase AH (Amano; immobilized), lipase P (Nagase), lipase AY from Candida rugosa, lipase G (Amano 50), lipase F (Amano F-AP15), lipase PS (Amano), lipase AH (Amano), lipase D (Amano), lipase AK from Pseudomonas fluorescens, lipase PS from Pseudomonas cepacia, newlase I from Rhizopus niveus, lipase PS-CI (immobilized lipase from Pseudomonas cepacia). These lipases may, as the person skilled in the art is aware, be used as cell-free enzyme extracts or else in the corresponding microorganism cell.
The proteases may also be commercially available, such as, for example, serine proteases such as subtilisins. The subtilisin may be savinase from Bacillus sp., alcalase, subtilisin from Bacillus licheniformis and also proteases from Aspergillus, Rhizopus, Streptomyces or Bacillus sp.
The biotechnological racemate resolution is expediently carried out at a temperature of from 10 to 80xc2x0 C. and at a pH of from 4 to 9.
The biotechnological racemate resolution is expediently carried out in a protic or aprotic organic solvent. Suitable aprotic organic solvents are ethers such as tert-butyl methyl ether, diisopropyl ether, dibutyl ether, dioxane and tetrahydrofuran, aliphatic hydrocarbons such as hexane, organic bases such as pyridine, and carboxylic esters such as ethyl acetate, and suitable protic organic solvents are the C1-6-alcohols already described, such as, for example, pentanol.
The (1S,4R)xe2x80x94 or (1R,4S)-1-amino-4-(hydroxy-methyl)-2-cyclopentene derivatives of the general formulae VII and VIII formed in accordance with the invention during the biotechnological racemate resolution are, depending on the desired target compound (aminoalcohol of the formula V or VI), hydrolyzed by chemical means to give the aminoalcohol of the formula V or VI. The chemical hydrolysis is expediently carried out in an aqueous basic solution or using a basic ion exchanger. The aqueous basic solution is preferably, as for the hydrolysis of the cyclopentene derivatives of the general formula III described above, an alkali metal hydroxide. The basic ion exchangers can, for example, be Dowex 1xc3x978(OHxe2x88x92) and Duolite A147.
The chemical racemate resolution is carried out using an optically active tartaric acid such as using D-(xe2x88x92)-tartaric acid or L-(+)-tartaric acid.
The racemate resolution with D-(xe2x88x92)-tartaric acid is expediently carried out by firstly reacting the racemic 1-amino-4-(hydroxymethyl)-2-cyclopentene with the D-(xe2x88x92)-tartaric acid in the presence of a lower aliphatic alcohol.
Suitable lower aliphatic alcohols are the same as those described above. Preference is given to using methanol. The reaction which leads to formation of the salt is usually carried out at temperature between 20xc2x0 C. and the reflux temperature of the solvent, preferably at the reflux temperature.
If desired, the 1-amino-4-(hydroxymethyl)-2-cyclopentene D-tartrate formed during the reaction can be further purified by recrystallization from a lower aliphatic alcohol such as methanol.
The racemate resolution with L-(+)-tartaric acid is expediently carried out as that with D-(xe2x88x92)-tartaric acid. I.e. the racemate resoluton with L-(+)-tartaric acid is likewise carried out in the presence of a lower aliphatic alcohol and at a temperature between 20xc2x0 C. and the reflux temperature of the solvent, preferably at the reflux temperature. After cooling, the (1S,4R)-1-amino-4-(hydroxymethyl)-2-cyclopentene L-hydrogentartrate crystallizes out.
The (1R,4S)-1-amino-4-(hydroxymethyl)-2-cyclopentene L-hydrogentartrate is present, in particular, in dissolved form in the mother liquor.
Isolation, further purification (liberation) and conversion to the corresponding salt of (1R,4S)xe2x80x94 or (1S,4R)-1-amino-4-(hydroxymethyl)-2-cyclopentene takes place with a base and subsequent acid treatment. Suitable bases are alkali metal alkoxides, alkali metal or alkaline earth metal carbonates, or alkali metal or alkaline earth metal hydroxides. The alkali metal alkoxides may be sodium or potassium alkoxides. The alkali metal carbonate may be potassium or sodium carbonate, potassium or sodium hydrogencarbonate, and the alkaline earth metal carbonate may be magnesium or calcium carbonate. The alkali metal hydroxide may be sodium or potassium hydroxide, and the alkaline earth metal hydroxide may be calcium hydroxide. Conversion to the corresponding salt usually takes place with a mineral acid such as with sulphuric acid, hydrochloric acid or phosphoric acid, preferably with hydrochloric acid.
(1R,4S)- or (1S,4R)-1-amino-4-(hydroxymethyl)-2-cyclopentene D-hydrogentartrate and (1R,4S)xe2x80x94 or (1S,4R)-1-amino-4-(hydroxymethyl)-2-cyclopentene L-hydrogentartrate are compounds unknown in the literature and are likewise provided by the invention.
Preference is given to carrying out the chemical racemate resolution with D-(+)-tartaric acid due to the higher performance, technical facility and more efficient racemate resolution.
As for the racemic aminoalcohol, it is of course also possible to react the optically active (1R,4S)xe2x80x94 or (1S,4R)-1-amino-4-(hydroxymethyl)-2-cyclopentenes with D-(xe2x88x92)- or L-(+)-tartaric acid to give the corresponding tartrates.
A further constituent of the present invention is the further conversion, the acylation, of the (1R,4S)xe2x80x94 or (1S,4R)-1-amino-4-(hydroxymethyl)-2-cyclopentenestogive the (1R,4S)-1-amino-4-(hydroxymethyl)-2-cyclopentene derivative of the general formula 
Here, the substituent R is as defined in the cyclopenten derivative of the general formula III.
The acylation can be carried out using a carbonyl halide of the general formula 
in which X is a halogen atom, and R is as defined above, or using a carboxylic anhydride of the general formula 
in which R is as defined above.
The halogen atom X may be F, Cl, Br or I. Preference is given to Cl or F.
Examples of carbonyl halides are: acetyl chloride, chloroacetyl chloride, butyryl chloride, isobutyryl chloride, phenylacetyl chloride, benzyl chloroformate, propionyl chloride, benzoyl chloride, alkyl chloroformate or tert-butyloxycarbonyl fluoride.
Examples of carboxylic anhydrides are: tertbutoxycarbonyl anhydride, butyric anhydride, acetic anhydride or propionic anhydride. The acylation is preferably carried out using a carboxylic anhydride, in particular using tert-butoxycarbonyl anhydride.
The acylation can be carried out without solvent or using an aprotic orqanic solvent. The acylation is expediently carried out in an aprotic organic solvent. Suitable aprotic organic solvents are, for example, pyridine, acetonitrile, dimethylformamide, diisopropyl ether, tetrahydrofuran, toluene, methylene chloride, N-methylpyrrolidone, triethylamine, chloroform, ethyl acetate, acetic anhydride and mixtures thereof The acylation is expediently carried out at a temperature of from xe2x88x9220 to 100xc2x0 C., preferably from 0 to 80xc2x0 C.
The further conversion according to the invention of (1R,4S); or (1S,4R)-1-amino-4-(hydroxymethyl)-2-cyclopentene D- or L-hydrogentartrate to (1S,4R)xe2x80x94 or (1R,4S)-4-(2-amino-6-chloro-9-H-purine-9-yl)-2-cyclopentenyl-1-methanol, or a salt thereof, of the formulae 
is carried out by reacting (1R,4S)xe2x80x94 or (1S,4R)-1-amino-4-(hydroxymethyl)-2-cyclopentene D- or L-hydrogentartrate the formula 
to give (1S,4R)xe2x80x94 or (1R,4S)-4-[(2-amino-6-chloro-5-formamido-4 -pyrimidinyl) amino]-2 cyclopentenyl-1-methanol of the formulae 
and then cyclizing the latter in a known manner to give the compounds according to formula VIII and IX.
N-(2-Amino-4,6-dichloropyrimidin-5-yl) formamide can be prepared according to WO 95/21 161.
The reaction is expediently carried out in the presence of a base. Suitable bases are the same as those previously described for liberating (1R,4S)xe2x80x94 or (1S,4R)xe2x80x94 1-amino-4-(hydroxymethyl)-2-cyclopentenes from the corresponding tartrate.
The reaction is expediently carried out in a protic solvent. The protic solvent may be lower aliphatic alcohols such as methanol, ethanol, propanol, isopropanol, butanol or isobutanol.
The (1S,4R)xe2x80x94 or (1R,4S)-4-[(2-amino-6-chloro-5-formamido-4-pyrimidinyl)amino]-2-cyclopentenyl-1-methanol of the formula XI or XII is then cyclized in a known manner according to WO 95/21 161 to give the end product according to Formula VIII or IX.
The cyclization is usually carried out dissolved in trialkyl orthoformate in the presence of a concentrated aqueous acid. The trialkyl orthoformates used may be, for example, trimethyl or triethyl orthoformate.
The aqueous acid may be, for example, hydrogen fluoride, sulphuric acid or methanesulphonic acid.
A further constituent of the invention is the overall process for the preparation of (1S,4R)-4-(2-amino-6-chloro-9-H-purine-9-yl)-2-cyclopentenyl-1-methanol, or salts thereof, of the formula XII starting from (xe2x88x92)-2-azabicyclo[2.2.1]hept-5-en-3-one or (xe2x88x92)-acyl-2-azabicyclo[2.2.1]hept-5-en-3-one of the formulae 
in which R is as defined above, by reduction with a metal hydride to give an aminoalcohol of the formula 
or to give a cyclopentene derivative of the general formula 
in which R is as defined above, which are then converted into the corresponding hydrohalide salts, and then reacted with N-(2-amino-4,6-dichloropyrimidin-5-yl)formamide of the formula 
to give (1S,4R)-4-[(2-amino-6-chloro-5-formamido-4-pyrimidinyl)amino]-2-cyclopentenyl-1-methanol of the formula 
and then the latter is cyclized in a known manner to give the compound of the formula 
This process variant has the advantage that the hydrohalide salts formed therein may be used as a crude mixture in the preparation of the product of the formula